42 Harrington et al.: Invasive Plants and the Green Industry INVASIVE PLANTS AND THE GREEN INDUSTRY By Robin A. Harrington1, Ronald Kujawski2, and H. Dennis P. Ryan3 Abstract. There are many motivations for introducing plant offices were recommending the planting of introduced species to areas outside their native range. Non-native species, such as autumn olive (Elaegnus umbellata) and plants can provide food, medicine, shelter, and ecosystem multiflora rose (Rosa multiflora), for wildlife food and cover. services, as well as aesthetic value. However, some species, Despite the many benefits provided by introduced species, such as Norway maple (Acer platanoides), Japanese barberry there has been growing concern recently among land (Berberis thunbergii), and Oriental bittersweet (Celastrus managers regarding the negative impacts when introduced orbiculatus), have escaped from cultivation, with severe species escape cultivation and become invasive in local ecological and economic consequences. The approval of a habitats. Invasive plant species pose a major threat to National Invasive Species Management Plan in June 2001 biodiversity, habitat quality, and ecosystem function. Invasive has major implications for future plant introductions and plants can also have devastating economic impacts, through has generated concern in the green industry. There is loss of revenue (reduced agricultural and silvicultural general agreement among plant professionals regarding (1) production in the presence of invasive species) and the high a need for education of industry people and client groups costs associated with control programs. The magnitude of on the issue of invasive species, (2) minimizing economic these impacts resulted in Executive Order 13112, issued by disruption to the nursery industry, and (3) requirements for President Clinton on February 3, 1999, directing all federal objective data to support listing of species as invasive. agencies to prevent and control introductions of invasive Nursery and landscape associations in several states have species. On January 18, 2001, the National Invasive Species already taken steps toward listing invasive species in Council approved a National Invasive Species Management cooperation with conservation organizations. This proactive Plan (www.invasivespecies.gov/council/nmp.shtml)), which has approach ensures industry representation and input in major implications for the future introduction of plant species developing policy and, when coupled with intense and that will in turn impact the horticulture industry. ongoing educational programs for industry people and their Our objectives are to (1) present an overview of the clients, could dramatically reduce the introduction and concerns that have led to the National Invasive Species spread of invasive species. Management Plan, (2) present examples of the impacts of Key Words. Invasive plants; Norway maple; Japanese three important nursery species when they invade natural barberry; Oriental bittersweet; Acer platanoides; Berberis and managed areas, and (3) propose strategies to address thunbergii; Celastrus orbiculatus. the concerns of the green industry regarding constraints on future plant introductions and distribution. WHAT IS AN INVASIVE? For centuries, there have been many motivations for human In any discussion of the negative impacts of invasive plant dispersal of plant species around the globe. We have long species, it is important to put the invasive species issue into recognized the importance of non-native plants in providing context. First of all, not all introduced species become invasive. food, medicine, shelter, and ecosystem services, as well as In fact, only a small proportion do. Williamson and Fitter (1996) cultural enjoyment (Ewel et al. 1999; Mack 2001). Introducing proposed the “Tens Rule” based on statistical analysis of a new plants has been a common cultural phenomenon since the number of British animals and plants. The rule states that, on beginning of time. In North America, the Spanish were the first average, one in ten species introduced (either intentionally or Europeans to introduce a new tree to the Americas, the peach. accidentally) will escape from cultivation. Only one in ten of The northern Europeans brought with them other food trees these escaped species will become naturalized and form self- such as apple, pear, and cherry. It wasn’t until the1860s that sustaining populations. In turn, only one in ten of these natural- serious effort went into collecting new plants for America. ized species will likely become invasive. However, because of the Some of the famous collectors of this period were Dr. George vast number of introductions over the years, even the relatively Hall, who worked for 15 years in China; Dr. Charles Sargent of small percentage of the total that become invasive has resulted in the Arnold Arboretum; and Professor William Clark of the a great number of invasive plant species. Massachusetts Agricultural College, who collected plants while One of the major exceptions to the Tens Rule, where the working with the Japanese to establish an agricultural college in chance of moving from one stage in the invasion process to Sapporo during 1876. Until recently, state and federal wildlife the next is much greater than 10%, includes plants intro- Journal of Arboriculture 29(1): January 2003 43 duced intentionally for cultivation. This fact is not surprising other intact forests (Webb and Kaunzinger 1993) where it because these species are selected to do well in the region reduces understory biodiversity (Wyckoff and Webb 1996). where they are introduced. Also, characteristics that make Norway maple has been overused as a street tree because it species desirable as street trees or ornamental plantings also was readily available when Dutch elm disease eliminated most increase the likelihood that they will become invasive. Urban American elms (Ulmus americana) as our principal city tree. and suburban sites typically require plants that will be able to Norway maple was and is inexpensive and, when planted in survive in harsh conditions. In many cases, the plants are put tough urban conditions it did well, resulting in attractive, tree- into a poor soil with low oxygen conditions. They are exposed lined streets. to rapid changes in air temperature, and the light conditions can be poor. A tree that can live in these conditions is a Japanese Barberry survivor and can live just about anywhere. Japanese barberry (Berberis thunbergii) is a thorny, perennial However, the success of an invasion is affected not only by shrub native to central and southern Japan (Ohwi 1965). It was the characteristics of the invader but also by the characteristics first planted in North America in the late 1800s. By the 1920s, of the site being invaded. There has been growing interest in the species was naturalized in New England and, by the middle developing the ability to predict which sites are most susceptible of the century, it was considered a pest (Silander and Klepeis to invasion. There is general agreement among managers and 1999). The primary reason it was planted, and continues to be scientists that invasion is facilitated by disturbance. Disturbance planted, is its attractive foliage and berries (Figure 1). In addition, increases plant invasions by providing suitable microsites for it is often planted as a hedge because its thorny branches and germination and seedling establishment and by increasing light bushy habit make an effective barrier (Ehrenfeld 1997). and nutrient availability that enhance seedling survival and growth (Orians 1986; Hobbs and Huenneke 1992). Invasive species are better able to take advantage of high resource availability than many native species. For example, germination and seedling survival of two invasive woody species, paperbark (Melaleuca quinquinerva) and Brazilian pepper (Schinus teribinthifolius), were greater in disturbed communities than in intact native communities (Ewel 1986). Opening up of forest canopies through management activities or natural disturbance increases light availability and often favors growth and survival of invasive species (Horvitz et al. 1998; Knapp 1998). Although natural disturbance has sometimes been implicated in the establishment of invasive species in natural areas (Horvitz et al. 1998), our increasing fragmentation of the landscape, especially at the suburban–rural interface, is responsible for the establish- ment and spread of a great number of invasive species. Recent observations of successful invasions in intact forest ecosystems indicate that although disturbance facilitates invasion, it is not a prerequisite. Certain shade-tolerant species have been able to establish in forest understories with dire consequences for silvicultural and natural area management. We will present three examples across a range of growth forms: a tree, a shrub, and a vine. We will describe the characteristics that make these invasive species desirable as street trees and ornamental plantings, which of these characteristics increase their invasiveness, and the consequences of their invasion in forest habitats. Norway Maple Norway maple (Acer platanoides) is a commonly planted street tree in the United States because of its ability to tolerate stressful urban environments, combined with its ease of propagation and a variety of desirable cultivars Figure 1. Attractive foliage and berries of Japanese (Nowak and Rowntree 1990). From this widespread barberry make this species a